I don't see why you couldn't use plants from ponds. Just as Carol said: rinse them well. And I'd add: watch out for snails. Maybe I'll collect some pond plants as well, just to see how they'd do in one of my tanks. It can also save you lots of money because if you want a large and fully planted tank, you can spend as much as $100-300 on aquatic plants when buying at a fish store or online. I checked aquarium plants to see approximately how much I'll need to spend on plants alone in order to have my 75 gallon tank fully planted. I'll probably buy my plants from that website and I'll have to spend anywhere between $100-300 (including shipping, which is very expensive because you want it delivered the next day, as you're dealing with live plants here).
Don't algae start forming in a tank because of favorable conditions in it (rather than being "transferred")? Especially when there are a lot of nutrients available in the water and there are not enough plants to compete with algae for these nutrients, or there are slow-growing plants that don't remove the nutrients fast enough. Otherwise, if there are a lot of plants that grow fast and consume all nutrients ... I don't see why algae should form, even if they're newly introduced with the pond plants. If a tank's water conditions are appropriate, the newly-introduced algae won't survive. I don't think algae grow like bacteria ... sort of "colonizing" themselves. They rather depend on nutrient availability. But I may be wrong ... just a thought.
True to a point. It won't start unless you have favorable conditions, but if its introduced, it can quickly grow out of control if not caught early, as the algae starts using up the nutrients. I've had it happen and fought for several months to get it under control once it had been introduced. The new algae throws off the balance that existed before it was brought into the picture. That's why most planted tank people I know of treat "dip" all plants before introducing them into their tanks. There are a couple of different dips that can be used, such as a bleach dip, alum or permanganate, but care has to be taken with each one and they all have drawbacks.
Algae usually shows up if the water parameters are out of whack. Plants are dipped for a lot of different reasons, for instance snails and parasites.
Now if algae is killing the plant and you put it in your tank then the plant dieing will kick the water parameters a skew and algae may show up
If you have a plant with algae on it by all means clean it off, but I don't think bringing plants in from the lake or rivers will cause a problem in home aquaria.